Elementary Reaction Steps That Precede or Follow a Unimolecular Reaction Step Can Obfuscate Interpretation of the Driving-Force Dependence to Its Rate Constant.

Assessing the validity of a driving-force-dependent kinetic theory for a unimolecular elementary reaction step is difficult when the observed reaction rate is strongly influenced by properties of the preceding or following elementary reaction step. A well-known example occurs for bimolecular reactions with weak orbital overlap, such as outer-sphere electron transfer, where bimolecular collisional encounters that precede a fast unimolecular electron-transfer step can limit the observed rate. A lesser-appreciated example occurs for bimolecular reactions with stronger orbital overlap, including many proton-transfer reactions, where equilibration of an endergonic unimolecular proton-transfer step results in a relatively small concentration of reaction products, thus slowing the rate of the following step such that it becomes rate limiting.

Single-Particle Measurements Reveal the Origin of Low Solar-to-Hydrogen Efficiency of Rh-Doped SrTiO Photocatalysts.

Solar-powered photochemical water splitting using suspensions of photocatalyst nanoparticles is an attractive route for economical production of green hydrogen. SrTiO-based photocatalysts have been intensely investigated due to their stability and recently demonstrated near-100% external quantum yield (EQY) for water splitting using wavelengths below 360 nm. To extend the optical absorption into the visible, SrTiO nanoparticles have been doped with various transition metals.

Mechanistic Insights on Permeation of Water over Iron Cations in Nanoporous Silicon Oxide Films for Selective H and O Evolution.

Electrocatalysts encapsulated by an ultrathin and semipermeable oxide layer offer a promising avenue for efficient, selective, and cost-effective production of hydrogen through photoelectrochemical water splitting. This architecture is especially attractive for Z-scheme water splitting, for which a nanoporous oxide film can be leveraged to mitigate undesired, yet kinetically facile, reactions involving redox shuttles, such as aqueous iron cations, by limiting transport of these species to catalytically active sites. In this work, molecular dynamics simulations were combined with electrochemical measurements to provide a mechanistic understanding of permeation of water and Fe(III)/Fe(II) redox shuttles through nanoporous SiO films.

Quantifying the Influence of Defects on Selectivity of Electrodes Encapsulated by Nanoscopic Silicon Oxide Overlayers.

Encapsulation of electrocatalysts and photocatalysts with semipermeable nanoscopic oxide overlayers that exhibit selective transport properties is an attractive approach to achieve high redox selectivity. However, defects within the overlayers─such as pinholes, cracks, or particle inclusions─may facilitate local high rates of parasitic reactions by creating pathways for facile transport of undesired reactants to exposed active sites. Scanning electrochemical microscopy (SECM) is an attractive method to determine the influence of defects on macroscopic performance metrics thanks to its ability to measure the relative rates of competing electrochemical reactions with high spatial resolution over the electrode.

Quantification of Excited-State Brønsted-Lowry Acidity of Weak Photoacids Using Steady-State Photoluminescence Spectroscopy and a Driving-Force-Dependent Kinetic Theory.

Photoacids and photobases constitute a class of molecules that upon absorption of light undergoes a reversible change in acidity, i.e. p.

Reinvigorating electrochemistry education.

Electrochemistry is an established discipline with modern frontiers spanning energy conversion and storage, neuroscience, and organic synthesis. In spite of the expanding opportunities for academic and industrial electrochemists, particularly in the growing energy-storage sector, rigorous training of electrochemists is generally lacking at academic institutions in the United States. In this perspective, we highlight the core concepts of electrochemistry and discuss ways in which it has been historically taught.

Reconciliation of Differences in Apparent Diffusion Coefficients Measured for Self-Exchange Electron Transfer between Molecules Anchored to Mesoporous Titanium Dioxide Thin Films.

Redox-active sites present at large concentrations as part of a solid support or dissolved as molecules in fluid solutions undergo reversible self-exchange electron-transfer reactions. These processes can be monitored using a variety of techniques. Chronoamperometry and cyclic voltammetry are common techniques used to interrogate this behavior for molecules bound to mesoporous thin films of wide-bandgap semiconductors and insulators.

Photoacid-Modified Nafion Membrane Morphology Determined by Resonant X-ray Scattering and Spectroscopy.

Covalent attachment of photoacid dye molecules to perfluorinated sulfonic acid membranes is a promising route to enable active light-driven ion pumps, but the complex relationship between chemical modification and morphology is not well understood in this class of functional materials. In this study we demonstrate the effect of bound photoacid dyes on phase-segregated membrane morphology. Resonant X-ray scattering near the sulfur K-edge reveals that introduction of photoacid dyes to the end of the ionomer side chains enhances phase segregation among ionomer domains, and the ionomer domain spacing increases with increasing amount of bound dye.

5-Methoxyquinoline Photobasicity Is Mediated by Water Oxidation.

A mechanism explaining the photobasicity of 5-methoxyquinoline (5-MeOQ) is proposed on the basis of nonadiabatic molecular dynamics simulations using time-dependent hybrid density functional theory (TDDFT) and fewest switches surface hopping (SH) and analysis of existing ultrafast spectroscopy experiments. According to the TDDFT-SH simulations, the rate-determining step is hole transfer from photoexcited 5-MeOQ to adjacent water molecules within ∼5 ps followed by rapid electron-coupled proton transfer and deactivation to the ground state. This fast redox-catalyzed proton transfer mechanism is consistent with simple thermodynamic arguments, correlated wavefunction calculations, and recent isotope substitution time-resolved fluorescence experiments.

Interfacial Electron Transfer of Ferrocene Immobilized onto Indium Tin Oxide through Covalent and Noncovalent Interactions.

The immobilization of molecular species onto electrodes presents a direct route to modifying surface properties with molecular fidelity. Conventional methods include direct covalent attachment and physisorption of pyrene-appended molecular compounds to electrodes with aromatic character through π-π interactions. A recently reported hybrid approach extends the synthetic flexibility of the latter to a broader range of electrode materials.

Direct observation of sequential oxidations of a titania-bound molecular proxy catalyst generated through illumination of molecular sensitizers.

Natural photosynthesis uses the energy in sunlight to oxidize or reduce reaction centres multiple times, therefore preparing each reaction centre for a multiple-electron-transfer reaction that will ultimately generate stable reaction products. This process relies on multiple chromophores per reaction centre to quickly generate the active state of the reaction centre and to outcompete deleterious charge recombination. Using a similar design principle, we report spectroscopic evidence for the generation of a twice-oxidized TiO-bound molecular proxy catalyst after low-intensity visible-light excitation of co-anchored molecular Ru(II)-polypyridyl dyes.

Observation of Photovoltaic Action from Photoacid-Modified Nafion Due to Light-Driven Ion Transport.

Replacing passive ion-exchange membranes, like Nafion, with membranes that use light to drive ion transport would allow membranes in photoelectrochemical technologies to serve in an active role. Toward this, we modified perfluorosulfonic acid ionomer membranes with organic pyrenol-based photoacid dyes to sensitize the membranes to visible light and initiate proton transport. Covalent modification of the membranes was achieved by reacting Nafion sulfonyl fluoride poly(perfluorosulfonyl fluoride) membranes with the photoacid 8-hydroxypyrene-1,3,6-tris(2-aminoethylsulfonamide).

Modeling, Simulation, and Implementation of Solar-Driven Water-Splitting Devices.

An integrated cell for the solar-driven splitting of water consists of multiple functional components and couples various photoelectrochemical (PEC) processes at different length and time scales. The overall solar-to-hydrogen (STH) conversion efficiency of such a system depends on the performance and materials properties of the individual components as well as on the component integration, overall device architecture, and system operating conditions. This Review focuses on the modeling- and simulation-guided development and implementation of solar-driven water-splitting prototypes from a holistic viewpoint that explores the various interplays between the components.

Use of bipolar membranes for maintaining steady-state pH gradients in membrane-supported, solar-driven water splitting.

A bipolar membrane can maintain a steady-state pH difference between the sites of oxidation and reduction in membrane-supported, solar-driven water-splitting systems without changing the overall thermodynamics required to split water. A commercially available bipolar membrane that can serve this purpose has been identified, its performance has been evaluated quantitatively, and is demonstrated to meet the requirements for this application. For effective utilization in integrated solar-driven water-splitting systems, such bipolar membranes must, however, be modified to simultaneously optimize their physical properties such as optical transparency, electronic conductivity and kinetics of water dissociation.

Photoelectrochemical behavior of n-type Si(111) electrodes coated with a single layer of graphene.

The behavior of graphene-coated n-type Si(111) photoanodes was compared to the behavior of H-terminated n-type Si(111) photoanodes in contact with aqueous K3[Fe(CN)6]/K4[Fe(CN)6] as well as in contact with a series of outer-sphere, one-electron redox couples in nonaqueous electrolytes. The n-Si/Graphene electrodes exhibited stable short-circuit photocurrent densities of over 10 mA cm(-2) for >1000 s of continuous operation in aqueous electrolytes, whereas n-Si-H electrodes yielded a nearly complete decay of the current density within ~100 s. The values of the open-circuit photovoltages and the flat-band potentials of the Si were a function of both the Fermi level of the graphene and the electrochemical potential of the electrolyte solution, indicating that the n-Si/Graphene did not form a buried junction with respect to the solution contact.

Increase in the coordination number of a cobalt porphyrin after photo-induced interfacial electron transfer into nanocrystalline TiO2.

Spectroscopic, electrochemical, and kinetic data provide compelling evidence for a coordination number increase initiated by interfacial electron transfer. Light excitation of Co(I)(meso-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin) anchored to a nanocrystalline TiO(2) thin film, abbreviated Co(I)P/TiO(2), immersed in an acetonitrile:pyridine electrolyte resulted in rapid excited state injection, k(inj) > 10(8) s(-1), to yield Co(II)P/TiO(2)(e(-)), followed by axial coordination of pyridine to the Co(II)P and hence an increase in coordination number from four to five. The formal oxidation state and coordination environment of the Co metalloporphyrin on TiO(2) were assigned through comparative studies in fluid solution as well as by comparisons to previously reported data.

Non-Nernstian two-electron transfer photocatalysis at metalloporphyrin-TiO2 interfaces.

A long-standing question in the photochemical sciences concerns how to integrate single-electron transfers to catalytic multielectron transfer reactions that produce useful chemical fuels. Here we provide a strategy for the two-electron formation of C-C bonds with molecular catalysts anchored to semiconductor nanocrystallites. The blue portion of the solar spectrum provides band gap excitation of the semiconductor while longer wavelengths of light initiate homolytic cleavage of metal-carbon bonds that, after interfacial charge transfer, restore the catalyst.

Characterization of photoinduced self-exchange reactions at molecule-semiconductor interfaces by transient polarization spectroscopy: lateral intermolecular energy and hole transfer across sensitized TiO2 thin films.

Transient anisotropy measurements are reported as a new spectroscopic tool for mechanistic characterization of photoinduced charge-transfer and energy-transfer self-exchange reactions at molecule-semiconductor interfaces. An anisotropic molecular subpopulation was generated by photoselection of randomly oriented Ru(II)-polypyridyl compounds, anchored to mesoscopic nanocrystalline TiO(2) or ZrO(2) thin films, with linearly polarized light. Subsequent characterization of the photoinduced dichromism change by visible absorption and photoluminescence spectroscopies on the nano- to millisecond time scale enabled the direct comparison of competitive processes: excited-state decay vs self-exchange energy transfer, or interfacial charge recombination vs self-exchange hole transfer.

Reaction of Ru(II) diazafluorenone compound with nanocrystalline TiO2 thin film.

The Ru(II) compounds [Ru(bpy)(2)(mcbH)](2+) and [Ru(bpy)(2)(dafo)](2+), bpy is 2,2'-bipyridine where mcbH is 3-(CO(2)H)-2,2'-bipyridine and dafo is 4,5-diazafluoren-9-one, were synthesized, characterized, and anchored to nanocrystalline mesoporous TiO(2) thin films for excited state and interfacial electron transfer studies. X-ray crystallographic studies of [Ru(bpy)(2)(mcbH)](PF(6))(Cl) revealed a long Ru-N distance to the unsubstituted pyridine ligand of mcbH. Reaction of [Ru(bpy)(2)(dafo)](2+) with TiO(2) thin films resulted in interfacial chemistry.

Direct observation of photodriven intermolecular hole transfer across TiO2 nanocrystallites: lateral self-exchange reactions and catalyst oxidation.

Photoselection of Ru(II)-polypyridyl sensitizers with polarized pulsed-light excitation, when anchored to TiO(2) nanocrystallites interconnected in a mesoporous thin film, results in an anisotropic distribution of excited sensitizers. Under conditions where excited-state sensitizers efficiently inject electrons into TiO(2), the resulting oxidized sensitizers exhibit an initial anisotropy in their absorption difference spectra. Over the course of the charge-separated lifetime for many sensitizers, the transient absorption anisotropy signal decays to nearly zero indicative of lateral self-exchange hole-transfer reactions at the interface.

Excited-state electron transfer from ruthenium-polypyridyl compounds to anatase TiO2 nanocrystallites: evidence for a Stark effect.

Photophysical studies were performed with [Ru(dtb)(2)(dcb)](PF(6))(2) and cis-Ru(dcb)(dnb)(NCS)(2,) where dtb is 4,4'-(C(CH(3))(3))(2)-2,2'-bipyridine, dcb is 4,4'-(COOH)(2)-2,2'-bipyridine, and dnb is 4,4'-(CH(3)(CH(2))(8))(2)-2,2'-bipyridine), anchored to anatase TiO(2) particles (∼15 nm in diameter) interconnected in a mesoporous, thin film (∼10 μm thick) immersed in Li(+)-containing acetonitrile electrolytes. Pulsed-laser excitation resulted in rapid, nonquantitative excited-state injection into TiO(2) with a rate constant that could not be time-resolved, k(inj) > 10(8) s(-1), to yield an interfacial charge-separated state. Return of this state to ground-state products displayed observation-wavelength-dependent kinetics due to charge recombination and a second process.

Stark effects after excited-state interfacial electron transfer at sensitized TiO(2) nanocrystallites.

Photophysical studies were performed with [Ru(dtb)(2)(dcb)](PF(6))(2) and cis-Ru(dcb)(dnb)(NCS)(2,) where dtb is 4,4'-(C(CH(3))(3))(2)-2,2'-bipyridine, dcb is 4,4'-(COOH)(2)-2,2'-bipyridine, and dnb is 4,4'-(CH(3)(CH(2))(8))(2)-2,2'-bipyridine), anchored to anatase TiO(2) particles ( approximately 15 nm in diameter) interconnected in a mesoporous, 10 mum thick film immersed in Li(+)-containing CH(3)CN electrolytes with iodide or phenothiazine donors. Pulsed-laser excitation resulted in rapid excited-state injection and donor oxidation to yield TiO(2)(e(-))s and oxidized donors, while the metal-to-ligand charge-transfer (MLCT) absorption spectrum of the Ru(II) coordination compounds differed from that which was initially excited. The spectral data were consistent with an underlying Stark effect and indicated that the surface electric field was not completely screened from the molecular sensitizer.

Photodriven spin change of Fe(II) benzimidazole compounds anchored to nanocrystalline TiO(2) thin films.

Ferrous tris-chelate compounds based on 2-(2'-pyridyl)benzimidazole (pybzim) have been prepared and characterized for studies of spin equilibria in fluid solution and when anchored to the surface of mesoporous nanocrystalline (anatase) TiO(2) and colloidal ZrO(2) thin films. The solid state structure of Fe(pybzim)(3)(ClO(4))(2).CH(3)CN.

Photodriven heterogeneous charge transfer with transition-metal compounds anchored to TiO2 semiconductor surfaces.

A critical review of light-driven interfacial charge-transfer reactions of transition-metal compounds anchored to mesoporous, nanocrystalline TiO2 (anatase) thin films is described. The review highlights molecular insights into metal-to-ligand charge transfer (MLCT) excited states, mechanisms of interfacial charge separation, inter- and intra-molecular electron transfer, and interfacial charge-recombination processes that have been garnered through various spectroscopic and electrochemical techniques. The relevance of these processes to optimization of solar-energy-conversion efficiencies is discussed (483 references).